1. Field of the Invention
The present invention generally relates to a magnetic recording medium, and more particularly relates to a magnetic recording medium having a ferromagnetic metal thin film as a magnetic recording layer on a non-magnetic substrate.
2. Description of the Prior Art
A conventional magnetic recording medium is provided in such a manner that a magnetic paint composed mainly of an acicular ferromagnetic powder such as gamma -ferric oxide (.gamma.-Fe.sub.2 O.sub.3, chromium dioxide CrO.sub.2, Fe-Co alloy powder or the like and a polymeric binder is coated on a non-magnetic substrate to form a magnetic recording layer.
Whereas, such a magnetic recording medium is worthwhile to be noticed in which a ferromagnetic metal thin film is formed on a non-magnetic substrate through the use of technique of vacuum evaporation, sputtering or ion plating and so on of ferromagnetic metal or alloy and this film is used as a magnetic recording layer. Since the magnetic recording medium of this ferromagnetic metal thin film type utilizes no binder, it can increase the packing density of magnetic material, and hence even if the magnetic recording layer is made thin, it is possible to obtain a magnetic recording medium having a high residual magnetic flux density. In addition, since the magnetic material layer thereof is formed as a thin layer, upon recording a short wavelength signal, it is also possible to provide a magnetic recording medium which is particularly excellent in sensitivity for such short wavelength signal and therefore it is quite desirable for high density magnetic recording.
As a method for forming a magnetic recording medium of such ferromagnetic metal thin film type, known in the prior art is a so-called oblique evaporation method. This is a method in which vapors of ferromagnetic metal such as Fe and Co are obliquely incident on a non-magnetic substrate, thereby depositing and growing it on the non-magnetic substrate. In accordance with this oblique evaporation method, it is possible to obtain a magnetic recording medium having a high coercive force. But, to obtain a high coercive force, vapor particle beams of such as Fe and Co have to be incident on the non-magnetic substrate with an incident angle in a range from 40.degree. to 80.degree., the deposition is poor in efficiency and hence productivity is low. But, unless the above obilique evaporation method is utilized, it is difficult to provide a magnetic recording medium having a high coercive force. Since, for example, cobalt has a high crystalline magnetic anisotropy, cobalt is appreciated as a magnetic material to provide a magnetic layer having a high coercive force. Nevertheless, if cobalt is merely evaporated, that is, a Co vapor beam is introduced to the major surface of a non-magnetic substrate substantially perpendicular so as to form a Co layer or film, the coercive force of that Co layer is less than 100 Oe. Thus such low coercive force as mentioned above is not suitable for use in the high density magnetic recording medium.
To form a magnetic material layer having a high coercive force independent of such above oblique evaporation method, there is proposed such a method that a magnetic metal layer is not directly formed on the non-magnetic substrate but instead, an under layer is formed between the non-magnetic substrate and the magnetic metal layer. As an example of the magnetic recording medium having an under layer such magnetic recording medium is known in which an under layer is formed by evaporating chromium as an under layer and cobalt is deposited on the under layer by the evaporation thereof. But, the magnetic characteristic of the magnetic recording medium thus constructed is apt to be influenced by the substrate temperature at which the magnetic metal layer is evaporated. For example, if a coercive force greater than 400 Oe is presented by utilizing cobalt having a thickness of 400 .ANG. as an upper layer, it is necessary to heat the substrate up to 300.degree. C. Accordingly, when a magnetic material layer having a coercive force more. than 400 Oe is formed on a non-magnetic substrate, it was difficult to use a polymeric film such as polyethylene terephthalate having poor heat resistance property as the non-magnetic substrate.